The problem, or not a problem in which ever way you look at it is, TV has changed so much over the past 20 years in the UK, and there is no way a Channel such as the BBC or ITV will commission a show for 13 episodes, such as was done in the 80's with the original 2 series. This is a problem for many, as we only get 6 episodes now of modern TV shows, but that does not give the viewer a chance to grow on the story lines and characters.
Auf Wiedersehen, Pet Series 1 and 2 will always be the greatest TV show in my mind, but I am biased as I run the Official Website. I do believe however, that Series 3, 4 and The Special are really great television, but as times have changed and we are so used to long series runs, that we didn't have chance to get the lads on a great run.
For me, Series 3, was just getting into a great run in Arizona, and then we had to say goodbye to 'The Lads'. We may never see long episode runs that were the norm in the good old 80's, and thank goodness for DVD's!